"Should Biomaterials be Smart and Scaffolds Instructive or the Other Way Around?"

Josep A. Planell, PhD
Institute for Bioengineering of Catalonia
Open University of Catalonia

Regenerative medicine is probably one of the most scientific promises and challenges in this initial part of the 21st Century. Different strategies of cell therapy or tissue engineering are being developed and scaffolds able to deliver signals to the biological environment play a leading role. The question arising is whether the biological functionality of the scaffolds depends directly upon the properties their constitutive biomaterials. This ability to produce signals and stimuli able to control cell fate is what is generally understood as the role of smart biomaterials. The requirements on the scaffolds depend on the specific tissue application and this means that constraints on the scaffold design will need to be considered (geometry, handability, suturability, mechanical properties, etc). The present talk tries to discuss, by means of two examples, whether physico-chemical properties of biomaterials, surface and bulk, are as relevant issues in scaffold production as fabrication and processing. 1-Numerous calcium phosphate–containing ceramics, glass ceramics, and biological glasses have been developed for the repair and reconstruction of the bony tissue. Bioactive glasses have shown to be promising materials for the skeletal system healing, especially in vivo, where they have induced a rapid and beneficial response. A majority of these bioactive glasses is SiO2-based exhibiting a range of low dissolution rates. In contrast, phosphate bioabsorbable glasses have the ability to solubilize completely, and their degradation time may vary from few days to several months. Such glasses are good candidates as reinforcement phase for biodegradable composite materials for the construction of scaffolds for tissue engineering. The incorporation of an inorganic phase into the polymeric matrix may enhance the mechanical integrity of the material, as well as its biological behavior, and can also modify the degradation mechanism of the polymer. Calcium phosphate (CaP) glasses are well suited for bone remodeling given that they possess a chemical composition close to that of the mineral phase of bone and that their solubility rate can be adjusted by controlling their chemical composition. The success of a 3D scaffold depends on several parameters that range from the macro- to the nanoscale. Macro- and microporosity, as well as interconnectivity, are of great importance in promoting tissue ingrowth, vascularization, and the delivery of nutrients throughout the newly formed tissue. The attachment and adhesion of the cells on the material surface are protein mediated processes, where factors such as surface chemistry, surface energy, and topography can affect the cell material response. In vitro studies have shown that the biological properties of this bioactive, biodegradable calcium phosphate glass/polylactic acid composite biomaterial promote bone marrow-derived endothelial progenitor cell (EPC) mobilization, differentiation and angiogenesis through the creation of a controlled bone healing-like microenvironment. The angiogenic response is triggered by biochemical and mechanical cues provided by the composite, which activate two synergistic cell signaling pathways: a biochemical one mediated by the calcium-sensing receptor and a mechanosensitive one regulated by non-muscle myosin II contraction. Together, these signals promote a synergistic response by activating EPCs-mediated VEGF and VEGFR-2 synthesis, which in turn promote progenitor cell homing, differentiation and tubulogenesis. These findings highlight the importance of controlling microenvironmental cues for stem/progenitor cell tissue engineering and offer exciting new therapeutical opportunities for biomaterial-based vascularisation approaches and clinical applications. This is the first time that a composite containing a bioactive, biodegradable glass has been proven to be directly involved in angiogenesis and differentiation of endothelial progenitors. The main advantages of this approach (when compared to others such as protein immobilization or growth factor release) are the simplicity of material fabrication, low cost and off-the-shelf availability, making it a very attractive strategy for clinical applications involving musculoskeletal repair. Related to osteogenesis, in vitro studies revealed the induction of Mesenchymal Stem Cells proliferation, migration and differentiation towards osteogenic lineage and promoting Alkaline Phosphatase activity and Collagen type I production at early times (3 an 7 days) in absence of osteogenic media. Also, the induction of mineralization is remarkable when there is a high extracellular calcium concentration (10 mM). In vivo studies have shown that the addition of calcium phosphate glass (G5) to PLA results in a composite material with higher angiogenic capacity than that of PLA only. 2-To develop tissue engineering strategies useful for repairing damage in the central nervous system (CNS) it is essential to design scaffolds that emulate the NSC niche and its tight control of neural cell genesis, growth, and differentiation. The aim of this study is to develop an artificial scaffold of polylactic acid (PLA) nanofibers to induce an environment that mimic embryonic radial glia organization and favors neuronal migration after a brain injury. For this purpose uncoated 3D electrospun random and aligned PLA nanofibers were used to study the behavior of neural cells from mice cerebral cortices in vitro and in vivo. Both, random and aligned fibers supported neural cells growth, but only aligned fibers permit neural cells invasion. Moreover, aligned fibers induce immature phenotypes in neuronal and glial cell cultures. Glial cells grown in aligned fibers showed bipolar shape and expressed the radial glia markers Nestin and BLbP, and the progenitor marker Pax6. On the other hand, neurons grown in aligned fibers were characterized by a decrease in the expression of β-III Tubullin, and an increase of neuron restricted progenitor marker, Tbr2 and the stem cell marker Sox2.
In the in vivo model, aligned PLA nanofibers implanted in the somatosensory cortex, were not encapsulated by a scar and did not seem to have elicited a foreign body reaction. It was also discovered that immature glial cells stained with Nestin, progenitor’s cells and blood vessels could penetrate into the aligned nanofibers after 7 days of implantation, indicating that the topography of the scaffold is playing a role in the migration of neural cells inside the injury. Our results suggest that aligned PLA nanofibres may mimic some of the physical and biochemical characteristics of the NSC niche. Its mechanical and surface properties may act synergistically in the modulation of bipotential and glial restricted progenitor and neuronal progenitor’s phenotypes, while topography may play an important role in angiogenesis in vivo. The conclusion would be that for a scaffold to be instructive, not only the biomaterials should show some smartness, but the 3D processing of the scaffold is also a pivotal issue.